[Data] Hard deprecate Datasource-related methods

[bveeramani]

Why are these changes needed?

We deprecated write_datasource and write-related Datasource methods in Ray 2.9. This PR hard deprecates them for Ray 2.10.

In specific, this PR hard deprecates Dataset.write_datasource and removes the following user-facing APIs:

• Datasource.on_write_start
• Datasource.write
• Datasource.on_write_completed
• Datasource.on_write_failed
• DummyOutputDatasource (renamed to DummyOutputDatasink)

If a user implements one of the removed methods, their program won't break. Instead, they'll receive a deprecation error when they call Dataset.write_datasource.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[bveeramani]

I moved this warning to read_datasource.

[c21]

LG